'The nastiest soils on Earth' are getting recognized as a bigger problem

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Acid sulfate soils are characterized by their orange hue and their tendency to kill surrounding vegetation and fish. Anders Johnson's extensive research along Sweden's coastline reveals the widespread presence of these soils, underscoring their significant impact on water quality and ecosystem health.

As human activities accelerate the emergence of new acid sulfate soils, it becomes crucial to understand the roles that geochemistry and microbiology play in this process to find effective mitigation strategies.

A new doctoral thesis provides valuable insights into the microbiology of acid sulfate soils, sometimes dubbed "the nastiest soils on Earth" because of the threat they pose to ecosystems.

Acid sulfate soils become active when sulfide-rich sediments, typically found in current or historical coastal areas, are exposed to oxygen either by natural or human activity. This triggers chemical reactions, sped up by the resident microbial communities, that lower the soil pH and release harmful acids and metals.

"When these areas experience a rainfall or snowmelt, the accumulated acid and dissolved metals are washed into nearby water bodies, potentially killing plants and fish given high enough concentrations," says Johnson, researcher and Doctor in Ecology at Linnaeus University.

While these soils release various heavy metals, aluminum is one to note. High concentrations of aluminum in waterways leads to fish kills, as the metal binds to the gills of fish and prevents them from getting oxygen from the water.

Causes fish kills

A major fish kill in northern Sweden was directly attributed to acid and metals released from acid sulfate soils. Additionally, some wetlands around Kalmar recently experienced unexplained fish kills that could possibly be linked to acid sulfate soils that we have identified nearby.

Johnson's dissertation is based on research along Sweden's 2000-kilometer coastline.

Sweden and Finland have the highest presence of boreal acid sulfate soils in the world. A report from Finland in 2002 found that acid sulfate soils account for more acid and metals released into the environment than all Finnish industry. The extent of these soils in Sweden is still not fully known, but Johnson's doctoral thesis provides valuable knowledge.

Researchers have known about large areas of these soils in northern Sweden. This research, which covers Sweden's 2000-kilometer coastline, has now expanded those known areas into southern Sweden as well.

"And while the field sizes of acid sulfate soils are generally smaller in southern Sweden, they have the tendency to become more acidic and release more acid and metals into the environment," says Johnson.

The sulfate-rich sediments that can turn into acid sulfate soils do so when they are exposed to oxygen by draining and drying. The process has become more frequent and will be an even bigger problem in the future because of human activities, Johnson explains.

"Acid sulfate soils in Sweden will become a much bigger problem as droughts become more frequent and as wetlands are drained for agriculture," Johnson explains.

How to mitigate acid sulfate soils

The Linnaeus University researchers are investigating the geochemical and microbiological processes that occur as acid sulfate soils develop, while searching for ways to mitigate the negative environmental effects. Current strategies are costly and not very effective. They involve either treating the soils with limestone to neutralize the acidification or re-submerging the areas to slow the process.

Looking at how the bacterial communities of these soils change during different treatments may provide new insights about how to limit acid creation, but more research is needed.

"The best strategy we know so far is to prevent these soils from oxidizing in the first place. Once the process starts, it is very, very difficult to stop. Having a better understanding of where these soils are located is one way to avoid disturbing them," Johnson says.

"Then, if they are disturbed or have already been disturbed, understanding how the complex geochemistry and microbiology interactions work together is vital to finding ways to reduce their impacts."

More information: Microbiology of boreal acid sulfate soils: Biogeochemical drivers of acidity generation and metals leaching, (2024). DOI: 10.15626/LUD.541.2024

Provided by Linnaeus University